(with acknowledgements to John Salt for providing penetration data
and to Wladimir Alfaro for information about the Tsetse action)

Amended 7 January
2016

The Molins Gun was a 57 mm automatic cannon built
and used in the UK during the Second World War. The origin of the
weapon was the six pounder (6 pdr) manually-loaded tank and anti-tank
gun developed in the UK and also adopted by the USA for the anti-tank
role.

THE TANK AND ANTI-TANK GUNS

The "6 pdr 7 cwt" (referring to the
notional weight of the projectile and the weight of the gun
- minus mounting - respectively) was first designed in 1938 as a replacement for the 2 pdr
(40 mm) tank/anti-tank gun. However, its introduction was delayed by the
Dunkirk retreat, which resulted in the loss of so much military
equipment that the decision was taken to continue the production of the
2 pdr for longer than planned, in order to rebuild the numbers of guns.
It was therefore not until 1942 that the gun started to appear in
service. It was adopted by the USA in 1941 and was also fitted to
several British tanks, including the Crusader III, Cavalier, Centaur 1,
Cromwell I to III, Valentine VIII to X and Churchill III, IV and X. It
was not declared obsolete until 1960.

The
gun was produced in two different barrel lengths, which affected the
muzzle velocities. Initial
British production (Mk 2 anti-tank, Mk 3 tank guns)
concentrated on the 43 calibre (245 cm) weapon, this relatively short
barrel normally being credited to production constraints (although more
recently it has been suggested that it was a deliberate decision to
make the gun more wieldy), but in the Mk 4 AT and
Mk 5 tank guns, the barrel was lengthened to 50 calibres (285 cm) in
order to increase the muzzle velocity. The US-made guns (designated
57mm M1) all had a 50 calibre barrel. Weight of the anti-tank gun was
1,140-1,300 kg including wheeled mounting.

57mm M1 US AT gun (can be distinguished from the British 6pdr Mk 4 by the lack of a muzzle brake and US-pattern
wheels) with acknowledgements to John Carlin

Several
different types of ammunition were developed for these guns, all of
them using the same 57 x 441R cartridge case. Initially, a plain (AP)
steel shot was used, the original loading being followed in service by
a higher-velocity loading. From October 1942 this was replaced by a
capped (APC) shot to improve the performance against face-hardened
armour, at the same time as a ballistic capped shot (APCBC) which was
heavier (and therefore had a reduced muzzle velocity) but which had an
improved long-range performance. An HE shell was also developed, as was
an APCR (AP composite rigid, with a tungsten core) but neither saw much
use. In June 1944, just in time for D-day, the APDS (armour piercing
discarding sabot) shot was introduced. This greatly improved the
penetration, at the cost of some loss in accuracy, but never entirely
replaced the APCBC. In addition, a "Littlejohn" squeezebore shot with
an emergent calibre of 42.6 mm was developed and tested in 1942, but not
adopted; it could penetrate 123 mm / 460 m / 30 degrees and 111 mm at 920 m.
The USA developed its own ammunition; the AP M70 and APC M85 (the
latter with a ballistic cap and containing 34 g of Explosive D,
initiated by a base fuze), but also made some use of the APDS. All of
the projectiles had tracers.

The performance of the service
ammunition takes some unravelling because most sources do not
distinguish between the L/43 and L/50 guns, nor between the different
AP loadings. It should also be noted that penetration depended on the
quality of the opposing armour, and that different nations had
different standards of measurement. However, the most realistic
figures, mainly from official sources with some interpolation to fill
the gaps, appear to be as shown below. These were measured against MQ
(machineable quality) armour and required that 80% of proof shot should
pass completely through the plate. The capped shot appears poor but had
an improved performance against face-hardened armour.

IN 43 CAL GUN
(Gun Mks 1-3)

WEIGHT KG

MUZZLE VELOCITY
M/SEC

PENETRATION AT 460M
and 30º in MM

PENETRATION AT 920M
and 30º in MM

Shot, AP

2.8

820

73

62

Shot, AP (HV)

2.8

850

81

68

Shot, APC, Mk 8T

2.8

850

72

63

Shot, APCBC, Mk 9T

3.2

790

68

60

IN 50 CAL GUN
(Gun Mks 4-5)

WEIGHT KG

MUZZLE VELOCITY
M/SEC

PENETRATION AT 460M
and 30º in MM

PENETRATION AT 920M
and 30º in MM

Shot, AP, Mks 1 to 7

2.8

890

82

71

Shot, APC, Mk 8T

2.8

890

78

67

Shot, APCBC, Mk 9T

3.2

830

87

73

Shot, APCR, Mk 1T

1.8

1,080

109

90

Shot, APDS, Mk 1T

1.47

1,235

131

117

Shell, HE Mk 10T

approx 3

820

-

-

US AP M70

2.85

850

81

64

US APCBC/HE M86

3.3

820

81

64

THE MOLINS GUNS

In order to meet a perceived need
for a rapid-reaction vehicle-mounted anti-tank gun, the notion was
developed of fitting the 6 pdr 7 cwt with an autoloading mechanism and
mounting it in a fast, wheeled tank destroyer. The Molins Machinery
Company (previously mainly noted for making cigarette manufacturing
machinery) duly designed and produced a suitable mechanism. By the time
it emerged, however, it had been realised that the 6 pdr was not
powerful enough to defeat the new Tiger tank so the British Army
dropped its requirement.

THE NAVAL MOLINS

Despite
this setback, the Molins gun was adopted by the RN and was fitted to
many MTBs in the power-operated Mark VII mounting. In this form it used
the shorter 43-calibre barrel of the Mk 2 AT gun, had a feed capacity
of six rounds in the autoloader - plus one in the breech and another
twelve in a ready-use rack on the mounting - and fired
semi-automatically only (the gunner had to press the trigger for each
shot) at a rate of about 40 rpm. It was probably the most powerful gun
fitted to naval craft of this class and was regarded as very
successful. Complete in its mounting, it weighed 1,747 kg, and the gun
was known to the RN as the 6 pdr QF Mk IIA. Nearly 600 of these guns
were made.

The RN developed its own loading
for the ammunition, consisting of a 2.7 kg HE shell fired at only 655
m/sec. The low velocity may be due to the fact that the RN specified a
flashless propellant charge, presumably because MTBs usually operated
at night.

In addition to this, ten LCS(L)2
were fitted with the Army's manually-loaded Mk 5 gun in a Valentine
tank turret to provide close-in gunfire support for amphibious
landings. These used the Army ammunition.

THE AIRCRAFT MOLINS

The RAF then became interested in
fitting the Molins Gun in the de Havilland Mosquito, to form an
airborne anti-tank weapon to replace the Hurricane IID which had been
equipped with a pair of Vickers 40mm Class S guns. The aircraft was
duly developed as the Mosquito FB Mk XVIII, popularly known as the
"Tsetse", but by this time the RAF had lost interest in the anti-tank
gun role so the aircraft were brought into service by Coastal Command
for anti-ship (and specifically anti-U-boat) purposes. The Tsetse, of
which about thirty were built, served with No.248 Squadron during 1944
and is credited with sinking a U-boat. More unusually, on 10 March 1944, four
Mosquito fighter-bombers of 248 Squadron, escorting two Tsetse, attacked the IJN
submarine I-29 off Cape Penas, Spain, as it was heading towards Lorient,
escorted by eight Junkers Ju-88C-6s from Zerstörergeschwader ZG 1 at Cazaux. In
the resulting battle one Ju-88 was shot down, reportedly by a Tsetse. The
submarine was undamaged.

The
Molins Aircraft Gun shown above, which was technically known to the RAF as the "QF 6 pdr
Class M Mark I with Auto Loader Mk III" was based on the long-barrelled
(50 calibre) gun. The gun weighed 487 kg (635 kg with autoloader) and
was fully automatic, with a rate of fire of about 55 rounds per minute.
The ammunition supply in the autoloader consisted of 21 rounds, held in
five racks of unequal length, plus two additional rounds in the
feedway. The rounds in each rack were fed by a combination of gravity
and a spring-loaded arm and each rack was moved into place in turn by
an electric motor. The gun normally used the plain AP shot (that is the
only one shown in photographs), so had a high muzzle velocity of 890
m/sec (2,920 fps). Against U-boat hulls, it was calculated that it
would be able to penetrate the hull when striking at an angle of 45
degrees or more, at a range of about 1,400 m, even through 60 cm of water.
The gun/aircraft combination was extremely accurate, achieving a hit
rate in training of 33% against tank-sized targets - compared with 5%
for rocket projectiles. The Tsetse was eventually withdrawn from
service when the RAF decided to use rocket projectiles for such roles
because, despite their relative lack of accuracy, these were more
suited to a variety of purposes and could easily be fitted, or removed,
as required.

The Molins Gun in the Mosquito FB
Mk XVIII was tested in the USA in 1945, in comparison with the nearest
US equivalent, the manually loaded 75 mm AN-Mk 5 in the PBJ-1H. This
comparison was more valid than the difference in calibre might suggest,
for the 75 x 350R ammunition used in the American gun (the same as was
used by the M4 tank gun in the Sherman) was about the same overall size
as the 57 x 441R, and the 6 pdr and 75 mm tank guns were effectively
interchangeable in the later British tanks. The Molins Gun impressed
the Americans with its performance and reliability and was considered
superior to the 75 mm as it could achieve a much higher rate of fire. It
was noted that fairly violent evasive action and 2.5 positive Gs did
not cause stoppages - which could not be said for manual loading! The
Americans recommended that the Molins autoloader could be considered as
suitable not just for conventional guns but also for recoilless weapons
and spin-stabilised rockets.